1. Field of the Invention
The present invention relates to a Y/C separation circuit. More specifically, the present invention relates to a Y/C separation circuit which is utilized in a video signal processing apparatus such as a television receiver and in which a luminance signal (Y signal) and a chrominance signal (C signal) are separated from a composite video signal by utilizing a 1H (horizontal scanning period) delay line.
2. Description of the Prior Art
In the past, as a 1H delay line for such a Y/C separation circuit, in general, a glass delay line or a CCD delay line was utilized; however, an insertion loss of such a kind of delay line is not constant, each having variation of the insertion loss. On the other hand, as an important item for a performance of the Y/C separation circuit, there is a separation degree of the luminance signal and the chrominance signal, that is, a depth of a comb characteristic.
Therefore, in a prior art, as shown in FIG. 1, in order to make amplitudes of the C signal component of the composite video signal passed through a 1H delay line 1 and the C signal component of the composite video signal not passed through the 1H delay line 1 be coincident to each other, in a manufacturing step of the video signal processing apparatus such as a television receiver, video tape recorder and etc., it is necessary to adjust a gain of a gain-variable amplifier 2. That is, in FIG. 1 prior art, the variation of an amplitude due to the variation of the insertion loss of the 1H delay line 1 is corrected by adjusting the gain of the gain-variable amplifier 2, and the composite video signal passed through the 1H delay line 1 and the composite video signal not passed through the 1H delay line 1 are inputted to an adder 3 and a subtracter 4, respectively, and the luminance signal and the chrominance signal are withdrawn from the adder 3 and the subtracter 4, respectively. Thus, in FIG. 1 prior art, it was necessary to adjust in advance the gain of the gain-variable amplifier 2.
Then, the same assignee as that of the present invention has proposed a technique for solving such a disadvantage of FIG. 1 prior art in an embodiment shown in U.S. Pat. No. 5,267,027 issued on Nov. 30, 1993. In this proposed prior art, as shown in FIG. 2, in order to make an operation of the Y/C separation circuit be stable at a point that a color burst signal included in the luminance signal outputted from the Y/C separation circuit becomes to be minimized, a color burst signal included in the composite video signal not passed through the 1H delay line 1 (or the composite video signal passed through the 1H delay line 1) and the color burst signal included in the luminance signal are multiplied by a multiplier 5, and a voltage-controlled gain-variable amplifier 6 is controlled by an output signal from the multiplier 5, whereby the amplitude can be automatically adjusted.
Thus, in FIG. 2 prior art, by detecting the amplitude of the color burst signal, the insertion loss amount of the 1H delay line or the like is automatically corrected, and therefore, in a case where a signal including a large number of noises and having a small burst signal level such as a signal at a time of a weak electric field, a signal obtained by reproducing the video tape recorder, and etc., it is difficult to surely detect the amplitude of the color burst signal. That is, in FIG. 2 prior art, there occurs a problem that the Y/C separation degree is dependent on the level of the input signal, S/N ratio and etc.
Furthermore, one example of a conventional Y/C separation circuit in which a 1H glass delay line is utilized is shown in FIG. 3. The Y/C separation circuit shown in FIG. 3 separates the luminance signal and the chrominance signal by utilizing a fact that a phase of the chrominance signal is inverted at every two lines. Therefore, in FIG. 3 prior art, a delay time of the 1H glass delay line 1' must be surely 1H (horizontal scanning period). However, in general, in the 1H glass delay line 1', a pair of supersonic transducers are provided on the opposite angles of a rectangular glass substrate, and an electric signal which is inputted to one supersonic transducer is withdrawn from the other supersonic transducer as an electric signal with 1H delay according to a surface form of the glass substrate between the transducers, and therefore, the variation occurs in delay time due to the irregularity of the working of the glass substrate.
Then, as shown in FIG. 3, the 1H glass delay line 1' is terminated by utilizing a variable inductance coil L, and by adjusting an inductance value of the variable inductance coil L, the variation of the delay time of the 1H glass delay line 1' is corrected.
However, in the prior art Y/C separation circuit shown in FIG. 3, since the variable inductance coil L is adjusted by a human work, and therefore, not only it takes a long time for adjusting the inductance value but also the variation in performance occurs for each product.
Then, the same assignee as that of the present invention has proposed a technique in which the 1H glass delay line 1 is constructed by utilizing a semiconductor variable inductance circuit (hereinafter, called as "gylator") which terminates at least one of an input end and an output end of the 1H glass delay line 1', instead of the variable inductance coil L, in an embodiment of the above described U.S. Pat. No. 5,267,027. Then, signals at the input end and the output end of the 1H glass delay line 1' are multiplied, and by controlling the gylator by a multiplication output, the variation of the delay time of the 1H glass delay line 1' can be automatically adjusted.
In addition, in an output signal from the 1H glass delay line, in general, a spurious component which is delayed by more than one horizontal scanning period exists other than a signal component which is delayed by one horizontal scanning period. Therefore, a spurious component occurs by passing the chrominance signal which exists in an effective video signal period through the 1H glass delay line, and the spurious component is added to the color burst signal, and therefore, a phase condition of the color burst signal is changed.
This will be described with referring to FIG. 4. FIG. 4 shows an input chrominance signal and an output chrominance signal to or from the 1H glass delay line, wherein FIG. 4(A) shows the input chrominance signal to the 1H glass delay line, and FIG. 4(B) shows the output chrominance signal from the 1H glass delay line. In FIG. 4, in a case where a signal (C1 or C3) having a low color saturation degree of a video signal is inputted to the 1H glass delay line 1', although a spurious component of the signal C1 or C3 is superposed on the color burst signal B2 or B4 as shown in FIG. 5(B), a level of the spurious component of the signal C1 or C3 is sufficiently lower than a level of the signal B2 or B4, and therefore, the influence due to the spurious component to the color burst signal B2 or B4 is small.
On the other hand, in a case where a signal C2 having a high color saturation degree of the video signal is inputted to the 1H glass delay line 1', a spurious component of the signal C2 is superposed on the color burst signal B3. At this time, a level of the spurious component due to the signal C2 is high, and the influence due to the spurious component to the signal B3 becomes large, and therefore, a phase of the signal B3 is changed. Accordingly, if the gylator is controlled by a signal obtained on the basis of the signal B3, a delay amount of the 1H glass delay line 1' becomes different from that of a normal state, and therefore, the Y/C separation degree becomes bad.
In addition, in the above, the influence due to the spurious component by the chrominance signal to the Y/C separation degree is described; however, a spurious component due to a luminance signal also affects the Y/C separation degree.
In addition, it is difficult to surely control the phase relationship of the color burst signal at a time of a signal including a large number noises such as a signal at a weak electric field, a signal obtained by reproducing the video tape recorder, and etc.